According to a new presentation, GDDR5X is going to provide twice the data rate per memory access. We're going to see it jump from the current 32B data per memory access, to 64B data per memory access. GDDR5 has hit a ceiling at around 7Gbps, but GDDR5X is going to have a much higher I/O rates of around 10-12Gbps.

It looks as though GDDR5X is going to be used well into 2018, while GDDR5 will most likely be kept for lower-end cards. I think we're going to see $50 - $300 cards with GDDR5, $300 - $500 cards (and possibly higher) with GDDR5X, and $500+ left for the higher-end HBM2 technology. The use of GDDR5X by AMD and NVIDIA is going to save much more money on R&D, because instead of moving over their entire next-gen GPU product lines, they can continue to use GDDR5X, which is not all that different to the current GDDR5 technology.